Multi-carrier communication apparatus

ABSTRACT

A multi-carrier communications apparatus which communicates between a first station and a second station as a destination station using a reference signal used for estimating a state of a channel, the multi-carrier communications apparatus comprising a multi-carrier modulator which modulates data on a plurality of subcarriers, a transmission unit configured to transmit the reference signal together with the data modulated to the destination station every subcarrier, and a control unit configured to control a reference signal transmission pattern according to a given communication condition, the reference signal transmission pattern including a transmission interval of the reference signal transmitted by the transmission unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2001-295107, filed Sep. 26,2001, the entire contents of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multi-carrier communication apparatusfor use in a multi-carrier transmission system that is used to estimatethe state of the channel using a reference signal.

2. Description of the Related Art

At present, the role of data transmission by radio is gaining more andmore importance, due to the popularization of cellular phones, wirelessLANs, and digital broadcast. In light of portability, ease ofinstallation and cost, radio data transmission is significantlyadvantageous over wired data transmission. On the other hand, as radiois affected by the state of the airwaves, this affects the state of thechannel used, thus also the communication quality. Communication can beseriously degraded, due to the phenomenon of multi-reflection radiopropagation (multi-path).

Multi-carrier transmission, in which data is modulated into pluralsubcarriers and then transmitted, is known as a system for reducing theinfluence of multi-path. OFDM (Orthogonal Frequency DivisionMultiplexing) is one such transmission system. On the other hand, tocope with the changes in the channel state, a method of estimating thestate of the channel has been developed. In this method, a knownreference signal, called a pilot signal, is inserted into the data (datasignal).

The above multi-carrier transmission and reference signal transmissionare effective for removing the influence suffered in the transmitteddata on the channel. However, if both transmissions are combined, anoptimum reference signal may be different every subcarrier. In the priorart, dynamic and appropriate control of the reference signal is notexecuted for each subcarrier. Accordingly, the communication quality maysignificantly degrade. Also, the redundant reference signal istransmitted, resulting in increasing the redundancy of the transmission.

BRIEF SUMMARY OF THE INVENTION

It is an object of the invention to provide a multi-carriercommunication apparatus capable of realizing high communication qualityirrespective of changes in the channel, while suppressing theunnecessary transmission of a reference signal.

According to an aspect of the present invention, there is provided amulti-carrier communications apparatus which communicates between afirst station and a second station as a destination station using areference signal used for estimating a state of a channel, themulti-carrier communications apparatus comprising: a multi-carriermodulator which modulates data on a plurality of subcarriers; atransmission unit configured to transmit the reference signal togetherwith the data modulated to the destination station every subcarrier; anda control unit configured to control a reference signal transmissionpattern according to a given communication condition, the referencesignal transmission pattern including a transmission interval of thereference signal transmitted by the transmission unit.

According to another aspect of the present invention, there is providedA multi-carrier communications apparatus which communicates between afirst station and a second station as a destination station using areference signal used for estimating a state of a channel, themulti-carrier communications apparatus comprising: a multi-carriermodulator provided in the first station to modulate data on a pluralityof subcarriers; a transmission unit provided in the first station andconfigured to transmit the reference signal together with the datamodulated to the destination station; and a control unit provided in thefirst station and configured to receive at least one of a communicationquality in transmitting data to the destination station, a response ofthe channel, and a degradation degree of the reference signal which issent from the destination station, and control a reference signaltransmission pattern according to at least one of the communicationquality, the response and the degradation degree of the referencesignal, the reference signal transmission pattern including atransmission interval of the reference signal transmitted by thetransmission unit.

According to another aspect of the present invention, there is provideda multi-carrier communications apparatus which communicates between afirst station and a second station as a destination station using areference signal used for estimating a state of a channel, multi-carriercommunications apparatus comprising: a multi-carrier modulator providedin the first station to modulate data on a plurality of subcarriers; atransmission unit provided in the first station and configured totransmit the reference signal together with the data modulated to thedestination station; and a control unit provided in the first stationand configured to detect at least one of a communication quality intransmitting data to the destination station, a response of the channel,and a degradation degree of the reference signal which is sent from thedestination station, and control a reference signal transmission patternaccording to at least one of the communication quality, the response andthe degradation degree of the reference signal, the reference signaltransmission pattern including a transmission interval of the referencesignal transmitted by the transmission unit.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a block diagram illustrating a multi-carrier transmissionsystem according to a first embodiment of the invention;

FIG. 2 is a view illustrating a reference signal transmission patternemployed in the embodiment;

FIG. 3 is a block diagram illustrating a multi-carrier transmissionsystem according to a second embodiment of the invention;

FIG. 4 is a block diagram illustrating a code error detector andreference signal transmission controller employed in the embodiments ofthe invention;

FIG. 5 is a block diagram illustrating a response estimator andreference signal transmission controller employed in the embodiments ofthe invention;

FIG. 6 is a block diagram illustrating a response estimator andreference signal transmission controller according to a modification ofthe embodiments of the invention;

FIG. 7 is a block diagram illustrating a reference signal degradationdetector and reference signal transmission controller employed in theembodiments of the invention;

FIG. 8 is a block diagram illustrating a reference signal degradationdetector, code error detector, response estimator and reference signaltransmission controller employed in the embodiments of the invention;

FIG. 9 is a view illustrating examples of subcarrier groups employed inanother embodiment of the invention;

FIG. 10 is a block diagram illustrating a code error detector andreference signal transmission controller employed for subcarrier groupsin yet another embodiment of the invention;

FIG. 11 is a block diagram illustrating a multi-carrier transmissionsystem using a processor, according to a further embodiment of theinvention; and

FIG. 12 is a flowchart useful in explaining the operation of the systemof FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will be described in detail with referenceto the accompanying drawings.

(First Embodiment)

FIG. 1 shows a multi-carrier transmission system according to a firstembodiment of the invention. As shown, the multi-carrier transmissionsystem comprises a station 1 (self-station) as a multi-carriercommunication apparatus according to the invention, and a destinationstation 2 that communicates with the self-station 1. In this embodiment,the transmission pattern of a reference signal, which is inserted in thedata transmitted from the station 1 to the station 2, is controlled onthe basis of the signal received by the station 2 (i.e., the signaltransmitted from the station 1 to the station 2).

Firstly, in the station 1, a reference signal and reference signaltransmission pattern data output from a reference signal transmissioncontroller 101, and transmission data output from a data source 102 areinput to a multi-carrier modulator 103. The multi-carrier modulator 103modulates the transmission data into plural subcarriers (multi-carrier)by multi-carrier modulation such as OFDM modulation.

In the multi-carrier modulator 103, a reference signal from thereference signal transmission controller 101 is superimposed upon thetransmission data on the basis of reference signal transmission patterninformation from the reference signal transmission controller 101. Thesignal output from the multi-carrier modulator 103 is input to atransmitter 104, where it is converted into a signal suitable forpropagation through a channel 105. The output signal is transmitted tothe station 2 from an antenna (not shown) via the channel 105.

On the other hand, in the station 2, the signal transmitted from thestation 1 via the channel 105 is received by an antenna (not shown). Thereceived signal supplied from the antenna is input to a receiver 106,where it is subjected to a process reverse to the process executed inthe transmitter 104 of the station 1. The signal output from thereceiver 106 is input to a multi-carrier demodulator 107, where it issubjected to multi-carrier demodulation. Thus, the received signal issubjected to a process reverse to the process executed in themulti-carrier modulator 103 of the station 1, whereby the data andreference signal are reproduced.

The signal output from the multi-carrier demodulator 107 is input to acode error detector 108, response estimator 109 and reference signaldegradation detector 110. The code error detector 108 detects thecommunication quality with which data is transmitted from the station 1to the station 2. The response estimator 109 estimates the response (theresponse characteristic of the channel 105). The reference signaldegradation detector 110 calculates the correlation between thepresently received reference signal and previously received referencesignal, thereby detecting the degree of degradation of the referencesignal.

The code error occurrence data output from the code error detector 108,response data output from the response estimator 109, reference signaldegradation data output from the reference signal degradation detector110, and transmission data output from a data source 112 are input to amodulator 113, where they are subjected to predetermined modulation. Thesignal output from the modulator 113 is input to a transmitter 114,where it is converted into a signal suitable for propagation through thechannel 115. The output signal is transmitted to the station 1 from anantenna (not shown) via the channel 115.

The signal thus transmitted from the station 2 to the station 1 isreceived by the antenna (not shown) of the station 1. The receivedsignal supplied from the antenna is input to a receiver 116, where it issubjected to a process reverse to the process executed in thetransmitter 114 of the station 2. The signal output from the receiver116 is input to a demodulator 117, where it is subjected topredetermined demodulation that is reverse to the process executed inthe modulator 113 of the station 2. As a result, data is reproduced. Thesignal output from the receiver 116 is also input to the referencesignal transmission controller 101. On the basis of the input signal,the controller 101 determines the transmission pattern that includes thetransmission interval of the reference signal. The specificconfiguration of the reference signal transmission controller 101 willbe described in detail later.

As described above, according to the embodiment, in the station 2 as thereference signal receiving side, the code error detector 108, responseestimator 109 and reference signal degradation detector 110 generatesignals, serving as setting indexes for setting the reference signaltransmission pattern, on the basis of the signal transmitted theretofrom the station 1 via the multi-carrier demodulator 107. These indexsignals are transmitted from the station 2 to the station 1, and areused to control the transmission pattern of the reference signal, i.e.,pilot signal, transmitted from the reference signal transmissioncontroller 101 of the station 1 to the station 2. As a result, thedegradation of the communication quality and the unnecessarytransmission of the reference signal can be suppressed.

The embodiment may be modified such that the data source 112 andmodulator 113 are omitted from the station 2. In this case, thedemodulator 117 is also omitted from the station 1. Further, the channel105 from the station 1 to the station 2 may be identical to or differfrom the channel 115 from the station 2 to the station 1. Theconfiguration of the embodiment is especially effective in the lattercase, i.e., in the case of an asymmetric channel (interactive channel).

(Specific Example of Criterion Signal Transmission Pattern)

FIG. 2 is an example of the reference signal transmission pattern. Inthis example, the multi-carrier communication system uses a signal offour subcarriers. In FIG. 2, numbers 1–4 are assigned to subcarriersSC1–SC4 in the order beginning from the lowest frequency. In theexample, suppose that the subcarriers SC1 and SC4 have relativelysuperior transmission characteristics, and the subcarriers SC2 and SC3have relatively inferior transmission characteristics.

In this case, the transmission pattern (transmission interval) of thereference signal P is set so that the reference signal transmissioninterval (T1) for the subcarriers SC1 and SC4 is longer than thereference signal transmission interval (T2) for the subcarriers SC2 andSC3. The control of the reference signal transmission pattern, inparticular, the reference signal transmission interval, for eachsubcarrier can suppress degradation of the communication quality and theunnecessary transmission of the reference signal.

(Second Embodiment)

FIG. 3 shows a multi-carrier transmission system according to a secondembodiment of the invention, which comprises, as in the firstembodiment, a station 1 (self-station) as a multi-carrier communicationapparatus according to the invention, and a destination station 2 thatcommunicates with the self-station 1. In this embodiment, thetransmission pattern of a reference signal, which is inserted in thedata transmitted from the station 1 to the station 2, is controlled onthe basis of the signal received by the station 2 (i.e., the signaltransmitted from the station 1 to the station 2).

Firstly, in the station 2, a predetermined reference signal output froma reference signal generator 301, and data output from a data source 302are input to a modulator 303, where they are subjected to predeterminedmodulation. The signal output from the modulator 303 is input to atransmitter 304, where it is converted into a signal suitable forpropagation through a channel 305 in a radio network. The output signalis transmitted to the station 1 from an antenna (not shown) via thechannel 305.

The signal transmitted from the station 2 to the station 1 is receivedby the station 1 via an antenna (not shown). The received signalsupplied from the antenna is input to a receiver 306, where it issubjected to a process reverse to the process executed in a transmitter304 incorporated in the station 2. The signal output from the receiver306 is input to a demodulator 307, where it is subjected topredetermined demodulation reverse to the process executed in amodulator 303 in the station 2. As a result, data reproduction isexecuted.

The signal output from the demodulator 307 is input to a code errordetector 308 for detecting a code error, response estimator 309 forestimating the response, and reference signal degradation detector 310for calculating the correlation between the presently received referencesignal and previously received reference signal, thereby detecting thedegree of degradation of the reference signal.

The code error occurrence data output from the code error detector 308,response data output from the response estimator 309, and referencesignal degradation data output from the reference signal degradationdetector 310 are input to a reference signal transmission controller311. On the basis of the input signals, the reference signaltransmission controller 311 determines a reference signal transmissionpattern including the transmission interval of the reference signal. Thespecific configuration of the reference signal transmission controller311 will be described in detail later.

The reference signal output form the reference signal transmissioncontroller 311 and the data output from a data source 312 are input to amulti-carrier modulator 313, where they are modulated into pluralsubcarriers (multi-carrier) by multi-carrier modulation. The signaloutput from the multi-carrier modulator 313 is input to a transmitter314, where it is converted into a signal suitable for propagationthrough a channel 315. The output signal is transmitted to the station 2from an antenna (not shown) via the channel 315.

On the other hand, in the station 2, the signal transmitted from thestation 1 via the channel 315 is received by an antenna (not shown). Thereceived signal supplied from the antenna is input to a receiver 316,where it is subjected to a process reverse to the process executed inthe transmitter 314 of the station 1. The signal output from thereceiver 316 is input to a multi-carrier demodulator 317, where it issubjected to multi-carrier demodulation, thereby reproducing data. Thisembodiment is especially effective if the channel 305 from the station 2to the station 1 is identical to or similar to the channel 315 from thestation 1 to the station 2, i.e., in the case of so-called symmetricalchannels.

Thus, according to the embodiment, in the station 1 as the referencesignal transmission side, the code error detector 308, responseestimator 309 and reference signal degradation detector 310 generatesignals, serving as setting indexes for setting the reference signaltransmission pattern, on the basis of the signal transmitted theretofrom the station 2 via the demodulator 307. The index signals are usedto control the transmission pattern of the reference signal, i.e., pilotsignal, transmitted from the reference signal transmission controller311 of the station 1 to the station 2. As a result, the degradation ofthe communication quality and the unnecessary transmission of thereference can be suppressed.

Furthermore, the control of the transmission pattern of the referencesignal using data acquired in the station 1 as the reference signaltransmission side enables the above-mentioned advantages to be obtainedwithout making the configuration of the station 2 complicated. As aresult, the station 2 can be made compact and light. This means thatthis embodiment is especially effective if it is used in a mobile radiocommunication system in which, for example, the station 1 is a basestation, and the station 2 is a mobile terminal (portable terminal).

(Code Error Detector and Criterion Signal Transmission Controller)

FIG. 4 shows a specific configuration example of the code error detectorand reference signal transmission controller. In this case, a shortreference signal transmission interval is assigned to those of a numberN of subcarriers, in which the frequency of occurrence of a code errorexceeds a threshold value. The configuration of FIG. 4 is assumed to beemployed in the second embodiment shown in FIG. 2.

In FIG. 4, a reference signal transmission controller 405 (correspondingto the reference signal transmission controller 311 of FIG. 3) comprisesthe same number of subcarrier code error computing units 402 andthreshold comparison units 403 as the number N of subcarriers, and areference signal transmission determining unit 404.

The code error occurrence data output from a code error detector 401(corresponding to the code error detector 310 of FIG. 3) is input toeach subcarrier code error computing unit 402, whereby the code errorrate (code error occurrence frequency) of each subcarrier is computed.The signal output from the subcarrier code error computing unit 402 isinput to the threshold comparison unit 403, where it is subjected to adetermination, using a threshold value, as to a code error rate. Inother words, whether or not the code error rate exceeds the thresholdvalue is determined. The signal output from the threshold comparisonunit 403 is input to the reference signal transmission determining unit404. If the code error rate in a certain subcarrier exceeds thethreshold value, the reference signal transmission determining unit 404determines reference signal transmission intervals for the subcarriersso that a short reference signal transmission interval (for example, T2in FIG. 2) is set for the certain subcarrier.

The threshold comparison units 403 may use a single threshold value forall the subcarriers, or may use different threshold values for differentsubcarriers, or may use a common threshold value for subcarriers fewerthan the number N of subcarriers. Further, the threshold value may be apreset fixed value or may be a dynamically variable value.

The determination of the reference signal transmission interval for eachsubcarrier in accordance with the frequency of occurrence of a codeerror can suppress the degradation of the communication quality and theunnecessary transmission of the reference signal.

The configuration shown in FIG. 4 is applicable to the embodiment shownin FIG. 1. In this case, the modulator 113, transmitter 114, channel115, receiver 116 and demodulator 117 shown in FIG. 1 are interposedbetween the code error detector 401 and reference signal transmissioncontroller 405. However, the reference signal transmission controller405 operates in the same manner as the above.

(First Examples of Transmission Channel Response Estimator and CriterionSignal Transmission Controller)

FIG. 5 shows first examples of the response estimator and referencesignal transmission controller. This configuration is used to set thereference signal transmission interval longer for those of the number Nof subcarriers, in which the response power exceeds the threshold value.The configuration of FIG. 5 is assumed to be applied to the embodimentof FIG. 3.

In FIG. 5, a reference signal transmission controller 505 (correspondingto the reference signal transmission controller 311 of FIG. 3) comprisesthe is same number of channel response power computing units 502 foreach subcarrier and threshold comparison units 503 as the number N ofsubcarriers, and a reference signal transmission determining unit 504.

The response data output from a response estimator 501 (corresponding toresponse estimator 309 in FIG. 3) is input to the channel response powercomputing unit 502, where response power is computed for eachsubcarrier. The signal output from the channel response power computingunit 502 is input to the threshold comparison unit 503, where it issubjected to a determination, using a threshold value, as to responsepower. In other words, whether or not the response power exceeds thethreshold value is determined. The signal output from the thresholdcomparison unit 503 is input to the reference signal transmissiondetermining unit 504. The reference signal transmission determining unit504 determines reference signal transmission intervals for thesubcarriers so that a long reference signal transmission interval is setfor the subcarrier in which the response power exceeds the thresholdvalue.

The threshold comparison units 503 may use a single threshold value forall the subcarriers, or may use different threshold values for differentsubcarriers, or may use a common threshold value for subcarriers fewerthan the number N of subcarriers. Further, the threshold value may be apreset fixed value or may be a dynamically variable value.

The determination of the reference signal transmission interval for eachsubcarrier in accordance with the response power can suppress thedegradation of the communication quality, and can reduce the number oftransmission intervals of the reference signal.

The configuration shown in FIG. 5 is also applicable to themulti-carrier transmission system shown in FIG. 1. In this case, themodulator 113, transmitter 114, channel 115, receiver 116 anddemodulator 117 shown in FIG. 1 are interposed between the responseestimator 501 and reference signal transmission controller 505. However,reference signal transmission control is executed in the same manner asthe above.

(Second Examples of Transmission Channel Response Estimator andCriterion Signal Transmission Controller)

FIG. 6 shows second examples of the response estimator and referencesignal transmission controller. This configuration is used to set thereference signal transmission interval longer for those of the number Nof subcarriers, in which the response power exceeds the threshold value.The configuration of FIG. 6 is assumed to be applied to the embodimentof FIG. 3.

In FIG. 6, a reference signal transmission controller 605 (correspondingto the reference signal transmission controller 311 of FIG. 3) comprisesthe same number of subcarrier correlation computing units 602 andthreshold comparison units 603 as the number N of subcarriers, and areference signal transmission determining unit 604.

The response data output from a response estimator 601 (corresponding toresponse estimator 309 in FIG. 3) is input to the subcarrier correlationcomputing unit 602 configured to compute a response correlation betweenthe subcarriers adjacent on the frequency axis. Concerning subcarriersat an end of the frequency axis, the definition of adjacent subcarrierswhose correlation is to be computed may be changed.

The correlation output from the subcarrier correlation computing unit602 is input to the threshold comparison unit 603, where a determinationas to whether or not the correlation exceeds the threshold value isexecuted. The signal output from the threshold comparison unit 603 isinput to the reference signal transmission determining unit 604. Thereference signal transmission determining unit 604 determines referencesignal transmission intervals for all the subcarriers so that a longreference signal transmission interval is set for the subcarriers atwhich the correlation exceeds a threshold value.

The threshold comparison units 603 may use a single threshold value forall the subcarriers, or may use different threshold values for differentsubcarriers, or may use a common threshold value for subcarriers fewerthan the number N of subcarriers. Further, the threshold value may be apreset fixed value or may be a dynamically variable value.

The determination of the reference signal transmission interval for eachsubcarrier in accordance with the response correlation of each pair ofthe subcarriers adjacent on the frequency axis can suppress thedegradation of the communication quality and the unnecessarytransmission of the reference signal.

If the configuration shown in FIG. 6 is applied to the multi-carriertransmission system shown in FIG. 1, the modulator 113, transmitter 114,channel 115, receiver 116 and demodulator 117 shown in FIG. 1 areinterposed between the response estimator 601 and reference signaltransmission controller 605. However, reference signal transmissioncontrol is executed in the same manner as the above.

(Criterion Signal Deterioration Detector and Criterion SignalTransmission Controller)

FIG. 7 shows a reference signal degradation detector and referencesignal transmission controller. This configuration is used to set thereference signal transmission interval longer for those of the number Nof subcarriers, in which the correlation of the previously receivedreference signal and currently received reference signal exceeds athreshold value. The configuration of FIG. 7 is assumed to be applied tothe embodiment of FIG. 3.

A reference signal degradation detector 705 (corresponding to thereference signal degradation detector 308 of FIG. 3) comprises areference signal memory 701 configured to store a previously receivedreference signal, and the same number of reference signal correlationcomputing units 702 as the number N of subcarriers, each computing unit702 computing the correlation between the previously received referencesignal stored in the reference signal memory 701 and the currentlyreceived reference signal. The demodulated reference signal output fromthe demodulator 307 shown in FIG. 3 is input to the reference signalmemory 701 and reference signal correlation computing units 702.

The previously received reference signal to be output from the referencesignal memory 701 is output therefrom by a predetermined period laterand input to the reference signal correlation computing units 702. Thepreviously received reference signal may be the latest receivedreference signal, or an estimated value calculated from plural referencesignals received lately. Each reference signal correlation computingunit 702 computes the correlation between the present reference signaloutput from the demodulator 307 and the previously received referencesignal output from the reference signal memory 701, and outputs it. Thecorrelation output from each reference signal correlation computing unit702 is input to a reference signal transmission controller 706.

The reference signal transmission controller 706 comprises the samenumber of threshold comparison units 704 as the number N of subcarriers,and a reference signal transmission determining unit 705. Thecorrelation output from each reference signal correlation computing unit702 is input to a corresponding threshold comparison unit 704, where itis determined whether or not the correlation exceeds a threshold value.The signal output from each threshold comparison unit 704 is input tothe reference signal transmission determining unit 705, where thereference signal transmission interval is determined for the subcarrierso that a long reference signal transmission interval is set for thesubcarrier in which the correlation exceeds the threshold value.

The threshold comparison units 704 may use a single threshold value forall the subcarriers, or may use different threshold values for differentsubcarriers, or may use a common threshold value for subcarriers fewerthan the number N of subcarriers. Further, the threshold value may be apreset fixed value or may be a dynamically variable value.

The determination of the reference signal transmission interval for eachsubcarrier in accordance with the response correlation of each pair ofthe subcarriers adjacent on the frequency axis can suppress thedegradation of the communication quality and the unnecessarytransmission of the reference signal.

The configuration shown in FIG. 7 is also applicable to the embodimentshown in FIG. 1. In this case, the modulator 113, transmitter 114,channel 115, receiver 116 and demodulator 117 shown in FIG. 1 areinterposed between the reference signal degradation detector 703 andreference signal transmission controller 706. However, reference signaltransmission control is executed in the same manner as the above.

(Code Error Detector, Transmission Channel Response Estimator, CriterionSignal Deterioration Detector and Criterion Signal TransmissionController)

FIG. 8 shows a code error detector, response estimator, reference signaldegradation detector and reference signal transmission controller. Thisconfiguration is used to set the reference signal transmission intervalshorter in accordance with the sum of comparison results of respectivethreshold values as to the frequency of occurrence of a code error, aresponse power, a response correlation, and a reference signaldegradation amount. FIG. 8 shows the portion of the system related to asubcarrier k, the system being assumed to be applied to the embodimentof FIG. 3.

In FIG. 8, a code error detector 801, response estimator 802 andreference signal degradation detector 803 operate in the same manner asthe code error detector 401 of FIG. 4, response estimator 501 of FIG. 5,and reference signal degradation detector 703 of FIG. 7, respectively. Asubcarrier code error rate computing unit 807, channel response powercomputing unit 808 and subcarrier correlation computing unit 809 operatein the same manner as the subcarrier code error rate computing unit 402of FIG. 4, channel response power computing unit 502 of FIG. 5 andsubcarrier correlation computing unit 602 of FIG. 6, respectively.

A threshold comparison unit 810 inputs code error occurrence data outputfrom the subcarrier code error rate computing unit 807, and executes adetermination using a threshold value on it. It is defined that thelower the code error rate, the larger value the comparison result has. Athreshold comparison unit 811 inputs response power data output from thechannel response power computing unit 808, and executes a determinationusing a threshold value on it. It is defined that the larger theresponse power, the larger value the comparison result has. A thresholdcomparison unit 812 inputs a correlation output from the subcarriercorrelation computing unit 809, and executes a determination using athreshold value on it. It is defined that the larger the correlation,the larger value the comparison result has. A threshold comparison unit813 inputs a correlation output from a reference signal correlationcomputing unit 805, and executes a determination using a threshold valueon it. It is defined that the larger the correlation, the larger valuethe comparison result has.

The thresholds of the threshold comparison units 810–813, whichcorrespond to all the subcarriers, may be the same value for all thesubcarriers, or may be different values for different subcarriers, ormay be a common value for subcarriers fewer than the number N ofsubcarriers. Further, the threshold value may be a preset fixed value ormay be a dynamically variable value.

A reference signal transmission determining unit 814 is supplied withthe comparison results of the threshold comparison units 810–813. Thereference signal transmission determining unit 814 computes the sum ofthe input comparison results, and determines the reference signaltransmission interval so that the greater the computation result, thelonger the reference signal transmission interval.

The determination of the reference signal transmission interval for eachsubcarrier, using a plurality of received data, can further reliablysuppress the degradation of the communication quality and theunnecessary transmission of the reference signal.

If the configuration shown in FIG. 8 is applied to the multi-carriertransmission system shown in FIG. 1, the modulator 113, transmitter 114,channel 115, receiver 116 and demodulator 117 shown in FIG. 1 areinterposed between the code error detector 801, response estimator 802and reference signal degradation detector 803, and the reference signaltransmission controller 806. However, reference signal transmissioncontrol is executed in the same manner as the above.

Further, in the configuration of FIG. 8, all the comparison resultsconcerning the frequency of occurrence of a code error, response power,response correlation and reference signal degradation amount. However,arbitrary two or more of the comparison results may be used.

(Third Embodiment)

Although in the above-described embodiments, the reference signalpattern (e.g., reference signal transmission interval) is controlled onthe basis of the communication quality (code error rate) or response(response power) of each subcarrier, it may be controlled on the basisof the communication quality or response of each of the subcarriergroups that each include plural subcarriers.

FIG. 9 shows a predetermined number of subcarrier groups, into which anumber N of subcarriers (N is set to 8 in this embodiment forfacilitating the description) are divided. Specifically, subcarriersSC1–SC5 form a subcarrier group A, and subcarriers SC6–SC8 form asubcarrier group B. The subcarriers may be divided in a preset manner,or may be dynamically divided on the basis of, for example, thecorrelation described referring to FIG. 6.

The reference signal transmission pattern is controlled in units ofsubcarrier groups on the basis of respective control indexes set for thesubcarrier groups A and B. As the control index, the communicationquality or response of a subcarrier selected from each of the subcarriergroups A and B, or the average communication quality or response ofplural subcarriers selected from each of the subcarrier groups A and Bcan be used.

FIG. 9 shows the former case. The communication quality or responsevalues of the subcarrier SC3 and SC7 are used as the control indexes ofthe subcarrier groups A and B, respectively.

The control of the reference signal transmission pattern in units ofsubcarrier groups reduces the amount of processing executed forreference signal transmission control, thereby suppressing degradationof the communication quality and the unnecessary transmission of thereference signal, with the circuit scale reduced.

FIG. 10 shows a reference signal transmission controller 1405 that usesa subcarrier group code error computing unit 1402 configured to computea code error in units of subcarrier groups. The reference signaltransmission controller 1405 determines the code error rate of each ofthe subcarrier groups 1, 2, . . . , N that each includes a predeterminednumber of subcarriers, thereby determining the reference signaltransmission interval in units of subcarrier groups. This means that itis not necessary to determine the reference signal transmission intervalof each of the large number of subcarriers, and accordingly, theprocessing time required for determining the reference signaltransmission interval can be reduced. The computing of the responsepower and correlation can also be executed in units of subcarriergroups, and the reference signal transmission interval can be determinedon the basis of each of the computing results.

FIG. 11 shows a multi-carrier transmission system in which stations 1and 2 can determine the reference signal transmission interval usingsoftware. In this system, processors 1101 and 1111 are provided in thestations 1 and 2, respectively. The processor 1101 of the station 1comprises an input port for receiving data from a signal source 1102, anoutput port for outputting data to a transmitter 1104, and a receiverport for inputting data received by a receiver 1116. The transmitter1104 of the station 1 transmits data to the station 2 via a channel1105.

The processor 1111 of the station 2 comprises a receiver port forinputting data received by a receiver 1106, an input port for receivingdata from a signal source 1112, and an output port for outputting datato a transmitter 1114.

Referring to FIG. 12, the operation of the multi-carrier transmissionsystem of FIG. 11 will be described.

When transmission data has been input from the signal source 1102 to theprocessor 1101 (S11), it is subjected to multi-carrier modulation (S12).The reference signal based on reference signal transmission pattern datais superimposed on the transmission data (S13). The transmission datawith the reference signal is transmitted to the station 2.

In the station 2, the transmission data from the station 1 is subjectedto multi-carrier demodulation (S14). The demodulated data is used asoutput data and is subjected to code error detection (S15), responseestimation (S16) and reference signal degradation detection (step S17).The results of the code error detection, response estimation andreference signal degradation detection are modulated together withtransmission data from the signal source 1112 (S18), and is transmittedto the station 1.

In the station 1, data from the station 2 is demodulated (S19). On thebasis of the results of the code error detection, response estimationand reference signal degradation detection, the reference signaltransmission pattern including the reference signal transmissioninterval is determined (S20). On the basis of the reference signaltransmission pattern, the reference signal transmission interval foreach subcarrier is determined. The control of the reference signaltransmission pattern, in particular, the reference signal transmissioninterval, in units of subcarriers can suppress degradation of thecommunication quality and the unnecessary transmission of the referencesignal.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A multi-carrier communication apparatus which communicates between afirst station and a second station as a destination station using areference signal used for estimating a state of a channel, themulti-carrier communications apparatus comprising: a multi-carriermodulator which modulates data on a plurality of subcarriers; atransmission unit configured to transmit the reference signal togetherwith the data modulated to the destination station on every subcarrier;and a control unit configured to control a reference signal transmissionpattern according to a given communication condition, the referencesignal transmission pattern including a transmission interval of thereference signal transmitted by the transmission unit; wherein thecontrol unit is further configured to control the reference signaltransmission pattern according to at least one of a communicationquality in transmitting data to the destination station, a response ofthe channel, and a degradation degree of the reference signal; andwherein the destination station includes a detection unit configured todetect at least one of the transmission quality, the response, and thedegradation degree of the reference signal, and a transmitter whichtransmits at least one of the transmission quality, the response and thedegradation degree of the reference signal to the control unit.
 2. Amulti-carrier communication apparatus which communicates between a firststation and a second station as a destination station using a referencesignal used for estimating a state of a channel, the multi-carriercommunications apparatus comprising: a multi-carrier modulator whichmodulates data on a plurality of subcarriers; a transmission unitconfigured to transmit the reference signal together with the datamodulated to the destination station on every subcarrier; and a controlunit configured to control a reference signal transmission patternaccording to a given communication condition, the reference signaltransmission pattern including a transmission interval of the referencesignal transmitted by the transmission unit; wherein the control unit isfurther configured to control the reference signal transmission patternaccording to at least one of a communication quality in transmittingdata to the destination station, a response of the channel, and adegradation degree of the reference signal; and wherein the subcarriersare grouped into a plurality of subcarrier groups, and the control unitincludes a computing unit configured to compute a rate of the code erroras the transmission quality every subcarrier group and a determinationunit configured to determine the reference signal transmission intervalevery subcarrier group according to the rate of the code error.
 3. Amulti-carrier communication apparatus which communicates between a firststation and a second station as a destination station using a referencesignal used for estimating a state of a channel, the multi-carriercommunications apparatus comprising: a multi-carrier modulator whichmodulates data on a plurality of subcarriers; a transmission unitconfigured to transmit the reference signal together with the datamodulated to the destination station on every subcarrier; and a controlunit configured to control a reference signal transmission patternaccording to a given communication condition, the reference signaltransmission pattern including a transmission interval of the referencesignal transmitted by the transmission unit; wherein the control unit isfurther configured to control the reference signal transmission patternaccording to at least one of a communication quality in transmittingdata to the destination station, a response of the channel, and adegradation degree of the reference signal; and wherein the control unitis further configured to detect a power of the response every subcarrieras the transmission response and control the reference signaltransmission pattern every subcarrier according to the power of theresponse.
 4. A multi-carrier communication apparatus according to claim3, wherein the control unit is configured to control the referencesignal transmission pattern every subcarrier so that the transmissioninterval increases as the transmission quality is superior.
 5. Amulti-carrier communication apparatus which communicates between a firststation and a second station as a destination station using a referencesignal used for estimating a state of a channel, the multi-carriercommunications apparatus comprising: a multi-carrier modulator whichmodulates data on a plurality of subcarriers; a transmission unitconfigured to transmit the reference signal together with the datamodulated to the destination station on every subcarrier; and a controlunit configured to control a reference signal transmission patternaccording to a given communication condition, the reference signaltransmission pattern including a transmission interval of the referencesignal transmitted by the transmission unit; wherein the control unit isfurther configured to control the reference signal transmission patternaccording to at least one of a communication quality in transmittingdata to the destination station, a response of the channel, and adegradation degree of the reference signal; and wherein the subcarriersare grouped into a plurality of subcarrier groups, and the control unitis configured to detect a power of the response every subcarrier groupand control the reference signal transmission pattern every subcarriergroup according to the power of the response.
 6. A multi-carriercommunication apparatus according to claim 5, wherein the control unitis configured to control the reference signal transmission pattern everysubcarrier so that the transmission interval increases as thetransmission quality is superior.
 7. A multi-carrier communicationapparatus which communicates between a first station and a secondstation as a destination station using a reference signal used forestimating a state of a channel, the multi-carrier communicationsapparatus comprising: a multi-carrier modulator which modulates data ona plurality of subcarriers; a transmission unit configured to transmitthe reference signal together with the data modulated to the destinationstation on every subcarrier; and a control unit configured to control areference signal transmission pattern according to a given communicationcondition, the reference signal transmission pattern including atransmission interval of the reference signal transmitted by thetransmission unit; wherein the control unit is further configured tocontrol the reference signal transmission pattern according to at leastone of a communication quality in transmitting data to the destinationstation, a response of the channel, and a degradation degree of thereference signal; and wherein the control unit is further configured touse a correlation between the responses corresponding to two of thesubcarriers, which are adjacent to each other on a frequency axis, asthe transmission response, and control the reference signal transmissionpattern every subcarrier according to the correlation.
 8. Amulti-carrier communication apparatus according to claim 7, wherein thecontrol unit is configured to control the reference signal transmissionpattern every subcarrier so that the transmission interval increases asthe correlation increases.
 9. A multi-carrier communication apparatuswhich communicates between a first station and a second station as adestination station using a reference signal used for estimating a stateof a channel, the multi-carrier communications apparatus comprising: amulti-carrier modulator which modulates data on a plurality ofsubcarriers; a transmission unit configured to transmit the referencesignal together with the data modulated to the destination station onevery subcarrier; and a control unit configured to control a referencesignal transmission pattern according to a given communicationcondition, the reference signal transmission pattern including atransmission interval of the reference signal transmitted by thetransmission unit; wherein the control unit is further configured tocontrol the reference signal transmission pattern according to at leastone of a communication quality in transmitting data to the destinationstation, a response of the channel, and a degradation degree of thereference signal; and wherein the control unit is further configured touse a correlation between a current reference signal and a pastreference signal as the degradation degree of the reference signal, andcontrol the reference signal transmission pattern every subcarrieraccording to the correlation.
 10. A multi-carrier communicationapparatus according to claim 9, wherein the control unit is configuredto control the reference signal transmission pattern every subcarrier sothat the transmission interval increases as the correlation increases.11. A multi-carrier communications apparatus which communicates betweena first station and a second station as a destination station using areference signal used for estimating a state of a channel, themulti-carrier communications apparatus comprising: a multi-carriermodulator provided in the first station to modulate data on a pluralityof subcarriers; a transmission unit provided in the first station andconfigured to transmit the reference signal together with the datamodulated to the destination station; and a control unit provided in thefirst station and configured to receive at least one of a communicationquality in transmitting data to the destination station, a response ofthe channel, and a degradation degree of the reference signal which issent from the destination station, and control a reference signaltransmission pattern according to at least one of the communicationquality, the response and the degradation degree of the referencesignal, the reference signal transmission pattern including atransmission interval of the reference signal transmitted by thetransmission unit.
 12. A multi-carrier communication apparatus accordingto claim 11, wherein the control unit includes a computing unitconfigured to compute a rate of a code error as the transmission qualityevery subcarrier and a determination unit configured to determine thereference signal transmission interval according to the rate of the codeerror.
 13. A multi-carrier communication apparatus according to claim11, wherein the subcarriers are grouped into a plurality of subcarriergroups, and the control unit includes a computing unit configured tocompute a rate of the code error as the transmission quality everysubcarrier group and a determination unit configured to determine thereference signal transmission interval every subcarrier group accordingto the rate of the code error.
 14. A multi-carrier communicationsapparatus which communicates between a first station and a secondstation as a destination station using a reference signal used forestimating a state of a channel, the multi-carrier communicationsapparatus comprising: a multi-carrier modulator provided in the firststation to modulate data on a plurality of subcarriers; a transmissionunit provided in the first station and configured to transmit thereference signal together with the data modulated to the destinationstation; and a control unit provided in the first station and configuredto detect at least one of a communication quality in transmitting datato the destination station, a response of the channel, and a degradationdegree of the reference signal which is sent from the destinationstation, and control a reference signal transmission pattern accordingto at least one of the communication quality, the response and thedegradation degree of the reference signal, the reference signaltransmission pattern including a transmission interval of the referencesignal transmitted by the transmission unit.
 15. A multi-carriercommunication apparatus according to claim 14, wherein the subcarriersare grouped into a plurality of subcarrier groups, and the control unitincludes a computing unit configured to compute a rate of the code erroras the transmission quality every subcarrier group and a determinationunit configured to determine the reference signal transmission intervalevery subcarrier group according to the rate of the code error.